Over the years, metallurgists engaged in material development for the utility industry have continually developed alloys meeting requirements for both high strength at elevated temperatures and corrosion resistance under severe environmental conditions. This quest for increasing performance is far from over as designers and engineers seek to increase productivity and efficiency, lower operating costs and extend service lives. All too often, researchers terminated their efforts when the target combination of properties was achieved, thereby leaving the optimization of the alloy range open for future exploitation. Such is the case, for example, in coal-fired, ultra-supercritical boiler materials in critical need of advanced alloys to maintain progress. This service requires ever-increasing strength at increasingly higher temperatures, as operating conditions become more demanding and service lives are required to be trouble-free over the life of the equipment. Coal-fired ultra-supercritical boiler designers must develop the materials meeting their advanced requirements as they improve efficiency by raising steam pressure and temperature.
Today's boilers with efficiencies around 45% typically operate up to 290 bar steam pressure and 580° C. steam temperature. Designers are setting their sights on 50% efficiency or better by raising the steam conditions as high as 325 bar/760° C. To meet this requirement in the boiler materials, the 100,000 hour stress rupture life must exceed 100 MPa at temperatures as high as 760° C. Additionally, raising steam temperature has made steam corrosion more troublesome placing a further requirement on any new alloy. This requirement is less than 2 mm of metal loss in 200,000 hours for steam oxidation in the temperature range of 700° C. to 800° C. For service as a header alloy, the material must be fabricable as thick-walled pipe (i.e., up to 80 mm wall thickness) and be fissure-free weldable into complex headers using conventional metal working and welding equipment. This places a major constraint on the fabricability and welding characteristics acceptable in manufacture and field installation. Such characteristics run counter to the need for superior strength in boiler tube service.
To meet the strength and temperature requirements of future ultra-supercritical boiler materials, designers must exclude the usual ferritic, solid solution austenitic and age-hardenable alloys heretofore employed for this service. These materials commonly lack one or more of the requirements of adequate strength, temperature capability and stability or steam corrosion resistance. For example, the typical age-hardenable alloy must be alloyed with insufficient chromium for oxidation resistance in order to maximize the age-hardening potential of the alloy, thereby developing high strength at elevated temperatures. However, adding chromium not only degrades the strengthening mechanism but, if added in excess, can result in embrittling sigma or alpha-chromium formation. Since 538° C. to 816° C. is a very active range for carbide precipitation and embrittling grain boundary film formation, alloy stability is compromised in many alloys in the interest of achieving high temperature strength and adequate steam oxidation resistance.